1. Field of the Invention
The invention relates to a method of driving a liquid crystal display device and a liquid crystal display device, which are adapted to change, with time, the gray level of a display signal to be fed to each picture element of a liquid crystal display panel, thereby preventing the occurrence of a phenomenon in which an image as viewed from any oblique angle undergoes degradation in its color reproducibility, as compared to that as viewed from the front.
2. Description of the Prior Art
Liquid crystal display devices have the advantages of being thin and lightweight and also allowing low-voltage drive and thus low power consumption, as compared to CRTs (cathode ray tubes). Thus, liquid crystal display devices are used in various types of electronic equipment, such as televisions, notebook-sized PCs (personal computers), desktop PCs, PDAs (personal digital assistants), and mobile telephones. In particular, an active matrix liquid crystal display device, which includes a TFT (thin film transistor) which is provided for each picture element (or subpixel) so as to act as a switching element, has high driving capabilities and thus exhibits excellent display characteristics comparable to those of a CRT. Thus, active matrix liquid crystal display devices have come into wide use in the fields where CRTs have been heretofore used, such as desktop PCs and televisions.
As shown in FIG. 1, a liquid crystal display device generally comprises two transparent substrates 10 and 20 which are disposed with spacers 31 in between and are bonded with a sealing compound 32, and liquid crystal 30 sealed in between the substrates 10 and 20. One substrate 10 includes a picture element electrode and a TFT, which are provided for each picture element. The other substrate 20 includes a color filter to be faced with each picture element electrode, and a common electrode common to every picture element. The color filters are of three types: red (R), green (G), and blue (B). One color filter out of the three is laid over each picture element. A pixel is composed of three picture elements, that is, red (R), green (G) and blue (B) picture elements, which are located adjacent to each other.
Hereinafter, the substrate having the picture element electrodes and the TFTs will be called a “TFT substrate”, and the substrate to be faced with the TFT substrate will be called an “opposite substrate”. A structure formed of the TFT substrate, the opposite substrate, and liquid crystal sealed in between the substrates is herein referred to as a “liquid crystal display panel”.
Typically, the TFT substrate 10 is larger than the opposite substrate 20 by the size of a connect terminal. Sheet polarizers 41 and 42 are disposed on both sides of a liquid crystal display panel 40 comprising the TFT substrate 10 and the opposite substrate 20. A backlight (not shown) is disposed under the liquid crystal display panel 40.
Heretofore, a TN (twisted nematic) liquid crystal display device has been widely used in which horizontal alignment liquid crystal (i.e., liquid crystal having positive anisotropy of dielectric constant) is sealed in between two substrates 10 and 20 so that liquid crystal molecules are oriented in a twisted fashion. However, the TN liquid crystal display device has the disadvantage of having poor viewing angle characteristics and thus causing considerable variations in the contrast or color tone of a screen as viewed from any oblique angle. Thus, MVA (multi-domain vertical alignment) liquid crystal display devices having good viewing angle characteristics are developed for practical use.
FIGS. 2A and 2B are schematic cross-sectional views showing an example of an MVA liquid crystal display device. The TFT substrate 10 and the opposite substrate 20 are disposed with spacers (not shown) in between, and vertical alignment liquid crystal 30 (i.e., liquid crystal having negative anisotropy of dielectric constant) is sealed in between the substrates 10 and 20. A plurality of bank-shaped projections 13 are formed on a picture element electrode 12 of the TFT substrate 10 so as to act as structures for defining domains. The surfaces of the picture element electrode 12 and the projections 13 are coated with a vertical alignment film 14 made of, for example, polyimide.
A plurality of bank-shaped projections 23 are also formed on a common electrode 22 of the opposite substrate 20 so as to act as structures for defining domains. The projections 23 are displaced obliquely relative to the projections 13 on the TFT substrate 10. The surfaces of the common electrode 22 and the projections 23 are also coated with a vertical alignment film 24 made of, for example, polyimide.
In the MVA liquid crystal display device, almost all liquid crystal molecules 30a are oriented perpendicularly to the substrate surfaces as shown in FIG. 2A, under conditions where no voltage is placed between the picture element electrode 12 and the common electrode 22. However, the liquid crystal molecules 30a near the projections 13 and 23 are oriented perpendicularly to the inclined surfaces of the projections 13 and 23.
When a predetermined voltage is placed between the picture element electrode 12 and the common electrode 22, the liquid crystal molecules 30a are oriented obliquely relative to the substrate surfaces under the effect of an electric field. In this instance, the liquid crystal molecules 30a are tilted in different directions on both sides of the projections 13 and 23 as shown in FIG. 2B, so that so-called multi-domain is achieved.
In the MVA liquid crystal display device, the liquid crystal molecules 30a are tilted in different directions on both sides of the projections 13 and 23 under the application of a voltage, as shown in FIG. 2B. This allows preventing light from leaking obliquely, thus achieving excellent viewing angle characteristics.
Although the description has been given with regard to the above-mentioned example in which the projections are used as the structures for defining domains, slits formed in the electrode or recesses (or grooves) formed in the substrate surface may be used as the structures for defining domains. Although the description has been given with reference to FIGS. 2A and 2B in which the structures for defining domains are formed on both the TFT substrate 10 and the opposite substrate 20, the structures for defining domains may be formed on one substrate, either the TFT substrate 10 or the opposite substrate 20.
FIG. 3 shows an example in which slits 12a are formed in the picture element electrode 12 on the TFT substrate 10 so as to act as structures for defining domains. Lines of electric force are generated obliquely toward the center of each slit 12a on the edges of each slit 12a, so that the liquid crystal molecules 30a are tilted in different directions on both sides of each slit 12a. This allows achieving multi-domain, thus improving viewing angle characteristics.
FIG. 4 is a plan view showing an example of an actual MVA liquid crystal display device. FIG. 4 shows one picture element region of the TFT substrate of the MVA liquid crystal display device.
A plurality of gate bus lines 51 extending horizontally and a plurality of data bus lines 55 extending vertically are disposed on the TFT substrate with respective predetermined pitches. The gate bus lines 51 and the data bus lines 55 partition the TFT substrate into rectangular regions, which are picture element regions. Auxiliary capacitance bus lines 52 are formed on the TFT substrate. The auxiliary capacitance bus lines 52 are disposed parallel to the gate bus lines 51 and cross the centers of the picture element regions. A first insulating film is formed between the gate bus lines 51 and auxiliary capacitance bus lines 52 and the data bus lines 55 so as to provide electrical isolation between the gate bus lines 51 and the data bus lines 55 and electrical isolation between the auxiliary capacitance bus lines 52 and the data bus lines 55.
A TFT 54, a picture element electrode 56, and an auxiliary capacitance electrode 53 are formed in each picture element region. The TFT 54 uses a part of the gate bus line 51 as a gate electrode. A drain electrode 54d of the TFT 54 is connected to the data bus line 55, and a source electrode 54s of the TFT 54 is located opposite to the drain electrode 54d with the gate bus line 51 in-between. The auxiliary capacitance electrode 53 is located opposite to the auxiliary capacitance bus line 52 with the first insulating film in-between.
The auxiliary capacitance electrode 53, the TFT 54, and the data bus line 55 are coated with a second insulating film, and the picture element electrode 56 is located on the second insulating film. The picture element electrode 56 is made of a transparent conductor such as ITO (Indium-Tin Oxide), and is electrically connected to the source electrode 54s of the TFT 54 and the auxiliary capacitance electrode 53 via contact holes 62a and 62b formed in the second insulating film. Two slits 56a extending obliquely are symmetrically formed in the picture element electrode 56. The surface of the picture element electrode 56 is coated with a vertical alignment film made of, for example, polyimide.
A black matrix, a color filter, and a common electrode are formed on the opposite substrate to be faced with the TFT substrate. Bank-shaped projections 71 extending parallel to the slits 56a are formed on the common electrode, as shown by the chain lines of FIG. 4. The projections 71 are displaced obliquely relative to the slits 56a in the picture element electrode 56. The surfaces of the common electrode and the projections 71 are coated with a vertical alignment film made of, for example, polyimide.
In the liquid crystal display device configured as mentioned above, placing a predetermined voltage between the picture element electrode 56 of the TFT substrate and the common electrode of the opposite substrate yields four domains having different orientations of liquid crystal molecules. The projections 71 and the slits 56a are used as the boundaries of the domains. This allows achieving good viewing angle characteristics.
Conventional MVA liquid crystal display devices exhibit better viewing angle characteristics than TN liquid crystal display devices. In the former, a phenomenon, however, occurs in which a screen becomes whitish when viewed from any oblique angle. FIG. 5 is a plot showing the transmittance (T) versus applied voltage (V) characteristics (hereinafter referred to simply as “T-V characteristics”) of a screen as viewed from the front and as viewed at an oblique angle of 60 degrees from above. In FIG. 5, the horizontal and vertical axes indicate an applied voltage (V) and transmittance (T), respectively. As shown in FIG. 5, when slightly higher voltages than a threshold voltage are applied to a picture element electrode (as shown by the circled part in FIG. 5), the transmittance of the screen as viewed from the oblique angle is higher than the transmittance of the screen as viewed from the front. When relatively higher voltages are applied, the transmittance of the screen as viewed from the oblique angle is lower than the transmittance of the screen as viewed from the front. When the screen is viewed from the oblique angle, such low transmittance leads to small luminance differences among red, green and blue picture elements, thus resulting in the occurrence of the phenomenon in which the screen becomes whitish, as mentioned above. This phenomenon is called “washing out”. Washing out occurs not only in MVA liquid crystal display devices, but also in TN liquid crystal display devices.
U.S. Pat. No. 4,840,460 presents the approach of subdividing each picture element into a plurality of sub picture elements and capacitively coupling the sub picture elements. In a liquid crystal display device adopting this approach, an electric potential is divided according to the capacitance ratio of each sub picture element, so that different voltages can be applied to the sub picture elements. Thus, a plurality of regions having T-V characteristics having different thresholds are apparently present in each picture element. The presence of a plurality of regions having T-V characteristics having different thresholds in each picture element, as mentioned above, allows preventing the occurrence of the phenomenon in which the transmittance of the screen as viewed from the oblique angle is higher than the transmittance of the screen as viewed from the front as shown in FIG. 5, thus preventing the occurrence of the phenomenon in which the screen becomes whitish (i.e., discoloring).
Japanese Patent No. 3076938 (Japanese Unexamined Patent Application Publication No. Hei 05-66412) discloses a liquid crystal display device including a picture element electrode which is subdivided into plural (e.g., four) sub picture element electrodes 81a to 81d, and control electrodes 82a to 82d which are disposed under the sub picture element electrodes 81a to 81d, respectively, with an insulating film in-between, as shown in FIG. 6. In the liquid crystal display device, the control electrodes 82a to 82d are of different sizes, and a display voltage is applied to each of the control electrodes 82a to 82d via a TFT 80. A control electrode 83 is also disposed between the adjacent electrodes of the sub picture element electrodes 81a to 81d in order to prevent light from leaking through between the adjacent electrodes of the sub picture element electrodes 81a to 81d. 
The approach of subdividing each picture element into a plurality of capacitively coupled sub picture elements for the purpose of improving display characteristics, as disclosed in the above patents, is called “HT (halftone gray scale) technique based on capacitive coupling”.
Japanese Unexamined Patent Application Publication No. 2001-75073 presents an approach for improving the viewing angle characteristics of a liquid crystal display device. For example, the approach involves applying a first voltage V1 to each picture element electrode for even-numbered frames, and applying a second voltage V2, which is about 0.5 to 1.5 V lower than the first voltage V1, to each picture element electrode for odd-numbered frames. Hereinafter, the application of the first voltage V1 to each picture element electrode will be called “bright display”, and the application of the second voltage V2 lower than the first voltage V1 will be called “dark display”. A pattern indicating the arrangement of bright display picture elements and dark display picture elements is herein referred to as a “bright and dark display pattern”.
The publication No. 2001-75073 presents a description as given below. For the odd-numbered frames, all picture elements undergo dark display as shown in FIG. 7A. For the even-numbered frames, all picture elements undergo bright display as shown in FIG. 7B. This publication also presents a description as given below. For the odd-numbered frames, picture elements connected to odd-numbered (e.g., the nth, (n+2)th, . . .) gate bus lines may undergo dark display, and picture elements connected to even-numbered (e.g., the (n+1)th, (n+3)th, . . .) gate bus lines may undergo bright display, as shown in FIG. 8A. For the even-numbered frames, the picture elements connected to the odd-numbered gate bus lines may undergo bright display, and the picture elements connected to the even-numbered gate bus lines may undergo dark display, as shown in FIG. 8B. In FIGS. 7A and 7B and FIGS. 8A and 8B, R, G and B denote red (R), green (G) and blue (B) picture elements, respectively.
The HT technique based on capacitive coupling, as mentioned above, uses the approach (i.e., space division) of subdividing each picture element into a plurality of regions and applying different voltages to the regions for the purpose of improving viewing angle characteristics, whereas the liquid crystal display device disclosed in the publication No. 2001-75073 adopts the approach of changing, with time, a voltage to be applied to each picture element electrode for the purpose of achieving the effect of the HT technique. Hereinafter, this approach will be called “HT technique based on time division”.
In order to prevent burn-in, liquid crystal display devices are typically adapted to change the polarity of a voltage (or a display signal) to be applied to each picture element electrode for each frame. In this case, transmittance under the application of a positive-polarity (or plus) voltage is slightly different from transmittance under the application of a negative-polarity (or minus) voltage. Thus, flicker occurs, for example when a voltage of positive polarity and a voltage of negative polarity are applied to all picture elements for odd-numbered frames and even-numbered frames, respectively. Thus, liquid crystal display devices are typically adapted to apply voltages of different polarities to picture elements located horizontally and vertically adjacent to each other and are further adapted to change the polarity of a voltage to be applied to each picture element for each frame, as shown in FIGS. 9A and 9B.
Hereinafter, a pattern indicating the polarity of a voltage to be applied to each picture element, as shown in each of FIGS. 9A and 9B, will be called a “polarity pattern”. The polarity pattern in which voltages of different polarities are applied to a horizontal arrangement of picture elements every one picture element and voltages of different polarities are applied to a vertical arrangement of picture elements every one picture element, for example as shown in each of FIGS. 9A and 9B, is herein referred to as a “polarity pattern with transverse 1-dot inversion and longitudinal 1-dot inversion”.
Japanese Unexamined Patent Application Publication No. Hei 08-171369 presents the approach of applying, by turns, voltages of different polarities to adjacent data bus lines for the purpose of reducing poor display, such as a transverse (or horizontal) luminance gradient, transverse crosstalk, and a longitudinal (or vertical) luminance gradient.
Japanese Unexamined Patent Application Publication No. 2003-337577 presents a liquid crystal display device which is adapted to select between a 1-dot inversion polarity pattern in which the polarity is reversed every one picture element and a 2-dot inversion polarity pattern in which the polarity is reversed every two picture elements, for example according to vertical frequencies and the presence or absence of flicker.
The HT technique based on capacitive coupling has the disadvantage of reducing an aperture ratio and thus providing insufficient brightness, because a voltage to be applied to each sub picture element electrode cannot be used to provide a desired orientation of liquid crystal molecules in a region between the sub picture element electrodes. This HT technique also has the disadvantage of increasing the likelihood of a short circuit occurring between the sub picture element electrodes and the control electrodes or between adjacent sub picture element electrodes, because of requiring a thin insulating film between the control electrodes and the picture element electrodes or a slit of narrow width between the sub picture element electrodes.
The HT technique based on time division, as disclosed in the above publication No. 2001-75073, does not have these disadvantages. However, experimental tests and research carried out by the inventors have shown that the technique disclosed in the publication No. 2001-75073, as applied to MVA liquid crystal display devices, cannot achieve its full effect.
Specifically, the publication No. 2001-75073 gives the definition of the voltage V1 as the voltage which provides desired brightness when the voltage V1 alone is applied to each picture element electrode, and gives the definition of the voltage V2 as the voltage which is lower than the voltage V1 by a predetermined value (e.g., about 0.5 to 1.5 V). Brightness under the application of these different voltages V1 and V2 alternating with each other to each picture element electrode should be lower than brightness under the application of the voltage V1 alone. In the publication No. 2001-75073, the brightness under the application of the voltages V1 and V2 alternating with each other to each picture element electrode is considered to be substantially the same as the brightness under the application of the voltage V1 alone to each picture element electrode, because there is little difference between the brightness under the application of the voltage V1 to each picture element electrode and the brightness under the application of the voltage V2 to each picture element electrode.
FIG. 10 is a plot showing the relation between a gray level difference between the voltages V1 and V2 for halftone display (127/255) and brightness as viewed from any oblique angle. In FIG. 10, the horizontal and vertical axes indicate the gray level difference between the voltages V1 and V2 and the brightness as viewed from any oblique angle, respectively. Generally, such a gray level difference that a luminance difference is unnoticeable, as described in the publication No. 2001-75073, lies between about 1/255 and 4/255. When the gray level difference falls outside this range, the luminance difference is fully recognizable. However, a gray level difference of about 1/255 to 4/255 causes little change in brightness, as can be seen from FIG. 10. A difference of at least 96-level or more gray scale must be set in order to achieve a sufficiently great reduction in the brightness as viewed from any oblique angle, while maintaining brightness as viewed from the front. In other words, the HT technique based on time division cannot achieve its effect and thus improve viewing angle characteristics, when the difference between the voltages V1 and V2 is such that the luminance difference is unnoticeable as described in the publication No. 2001-75073.
In order that the HT technique based on time division may fully achieve the effect of improving viewing angle characteristics, the brightness under the application of the voltage V1 and the brightness under the application of the voltage V2 must be higher and lower than desired brightness, respectively, so as to increase the luminance difference between the voltages V1 and V2.
FIG. 11 is a plot showing the input gray level versus output gray level characteristics of the voltages V1 and V2 required to obtain desired brightness. In FIG. 11, the horizontal and vertical axes indicate the input gray level and the output gray level, respectively. For example in order that the output gray level will be 125/255, the input gray level for the voltage V1 and the input gray level for the voltage V2 must be set to 225/255 and 100/255, respectively, so as to produce a difference of as much as 125-level gray scale. Such a large difference between the voltages V1 and V2 causes a severe flicker in full screen, when the bright and dark display patterns shown in FIGS. 7A and 7B are used to drive the liquid crystal display panel. This large difference causes severe flickers in the form of transversely extending lines, when the bright and dark display patterns shown in FIGS. 8A and 8B are used to drive the liquid crystal display panel.
In short, the technique disclosed in the publication No. 2001-75073 has at least three problems as given below:    (1) an input signal does not match output brightness;    (2) the technique does not achieve the effect of improving viewing angle characteristics, when there is little gray level difference; and    (3) the bright and dark display pattern is noticeable, when there is a large gray level difference.
Japanese Patent Application No. 2003-93793 filed by the applicant gives a description with regard to the approach of applying, by turns, the voltages V1 and V2 to picture elements located horizontally and vertically adjacent to each other, as shown in FIGS. 12A and 12B. Bright and dark display patterns shown in FIGS. 12A and 12B (e.g., bright and dark display patterns with transverse 1-dot inversion and longitudinal 1-dot inversion) are combined with polarity patterns shown in FIGS. 12C and 12D (e.g., polarity patterns with transverse 1-dot inversion and longitudinal 1-dot inversion) to form patterns shown in FIGS. 12E and 12F, respectively. Specifically, dark display picture elements alternate with bright display picture elements in horizontal and vertical directions, and moreover, picture elements to be subjected to a voltage of positive polarity alternate with picture elements to be subjected to a voltage of negative polarity in the horizontal and vertical directions.
In this case, the negative-polarity voltage alone and the positive-polarity voltage alone, however, are applied to the bright display picture elements and the dark display picture elements, respectively, as can be seen from FIGS. 12E and 12F. This leads to the application of a direct-current component to a liquid crystal layer, and thus results in the occurrence of burn-in or flicker.
The application No. 2003-93793 also gives a description with regard to the approach of using polarity patterns shown in FIGS. 13C and 13D (e.g., polarity patterns with transverse 1-dot inversion and longitudinal 2-dot inversion) to drive the liquid crystal display panel. The polarity patterns shown in FIGS. 13C and 13D are combined with bright and dark display patterns shown in FIGS. 13A and 13B (e.g., bright and dark display patterns with transverse 1-dot inversion and longitudinal 1-dot inversion) to form patterns shown in FIGS. 13E and 13F, respectively. Specifically, bright display picture elements alternate with dark display picture elements in the horizontal and vertical directions. Moreover, picture elements to be subjected to a positive-polarity voltage and picture elements to be subjected to a negative-polarity voltage alternate with each other in the horizontal direction, and are arranged in the vertical direction so that the polarity changes every two picture elements.
In this case, however, only the voltage of one polarity (i.e., positive or negative polarity) is applied to the bright display picture elements of the horizontally arranged picture elements, and only the voltage of the other polarity (i.e., negative or positive polarity) is applied to the dark display picture elements thereof, as can be seen from FIGS. 13E and 13F.
The picture element electrodes of the horizontally arranged picture elements are connected to one and the same gate bus line via TFTs, and are capacitively coupled to one and the same auxiliary capacitance bus line to form auxiliary capacitance. Thus, a considerable imbalance in electric potential in the picture elements connected to one gate bus line, as shown in FIGS. 13E and 13F, causes variations in the electric potentials of the gate bus line and the auxiliary capacitance bus line. This causes flicker in each transverse line.
The above application No. 2003-93793 also presents the approach of using combinations of bright and dark display patterns shown in FIGS. 14A and 14B (e.g., bright and dark display patterns with transverse 2-dot inversion and longitudinal 1-dot inversion) and polarity patterns shown in FIGS. 14C and 14D (e.g., polarity patterns with transverse 1-dot inversion and longitudinal 2-dot inversion) for driving as shown in FIGS. 14E and 14F. As can be seen from FIGS. 14E and 14F, this approach achieves a spatial balance between bright display and dark display and a spatial balance between a positive polarity and a negative polarity. From the viewpoint of each picture element, this approach also changes the display state for each frame so that the picture element undergoes positive-polarity dark display, then positive-polarity bright display, then negative-polarity dark display, and then negative-polarity bright display. Therefore, this approach can prevent the occurrence of burn-in or flicker.
However, the approach shown in FIGS. 14A to 14F has the disadvantage of producing a coarse screen, because of changing bright display and dark display every two horizontal picture elements.